Currently, if people want to use their cell phones abroad, they generally need to activate a roaming service to enjoy an offshore voice communication service. In this case, an additional roaming fee is charged when the cell phone is used abroad.
In fact, the offshore communication service can be implemented by the VoIP technology, in which an analog signal (voice) is digitalized and transmitted over an IP network in real time in the form of data packet. However, this VoIP technology has the following problems: 1) when a cell phone user is in abroad and calls back to his/her domestic country using this technology, the telephone number of the calling party will not be displayed accurately on the called party side, thus causing confusion in identifying the calling party identity, thereby producing unnecessary difficulties in the communication between the both parties; 2) it may be possible that a user in abroad cannot be contacted by dialing the commonly-used number directly; and 3) short messages cannot be sent and received through the current VoIP technical solution.
Furthermore, in a case of a weak wireless signal, unstable network coverage or a base station failure, the cell phone cannot access to the network, which affects the normal communication of the user.